The present invention relates to an improved burner and burner method used, for example, in the production of asphalt and, in particular, to a burner of greatly simplified construction which promotes more complete mixing of fuel and air and which utilizes a unique combination of primary and boost gas injection when the burner uses gaseous fuel.
A combination fuel aggregate dryer burner with a flame shaping swirl mechanism is disclosed in U.S. Pat. No. 4,559,009. This burner describes the use of internal recirculation to dispense with the need for ceramic tile with the use of internal recirculation. A blower supplies all of the combustion air for the burner. With the burning of oil (i.e., firing "on oil"), the burner atomizer assembly divides primary air into two flows and imparts a high degree of swirl to the inner primary air flow. A continuous sheet of oil is blast-atomized into this swirling air flow and is immediately broken up into droplets entrained within the flow. This highly swirling inner primary air is swirled out against the less swirled outer primary air with resultant shear atomization. The ignited swirling fuel oil/air mixture moves axially downstream and radially outwardly to decrease axial pressure and promote upstream recirculation of burning and unburned gases. When the burner is firing "on gas" (i.e. is burning only gas as the fuel) or a combination of oil and gas, the gas itself is not swirled but is mingled with outwardly swirling primary and secondary air flows.
Another type of fuel burner for drying aggregate in the making of asphalt and the like, and configured to burn liquid fuels or gas is shown in U.S. Pat. No. 4,298,337. This type of burner is known in the industry as a 30% burner because a blower provides about 30% of the combustion air. In order to obtain a large turn-down ratio, it is necessary to provide compressed air to the atomizer to maintain a constant pressure even when oil flow and air from the turbo blower are operated at substantially reduced inputs. Instead of bluff body recirculation to achieve flame stabilization, the burner is described as utilizing internal recirculation through the use of atomized liquid fuel being mixed with air caused to swirl by a fixed swirl plate and a frusto-conical flame stabilization cone whose smaller end is spaced from the outlet of the burner cone to leave an annular inlet space. This arrangement is described as creating a low pressure zone near the center with a small, stable combustion volume. In the event gaseous fuel is used in lieu of oil, the gas also flows through the swirl plate blades to mix with the pressurized air from the blower as both the air and gas pass through the swirl plate.
Many other burners are also currently available or known for the combustion of gas, liquid fuel and combinations thereof. Typical burner constructions are shown in U.S. Pat. Nos. 3,163,203; 3,217,779; 3,391,981; 4,441,879; 4,451,230; 4,717,332; 4,859,173; and 5,009,174.
It is the goal of all these burners to provide a compact and efficient combustion burner, large turn-down ratio, flame stability, switchability between fuels, dependable operation and economical manufacture. The combustion burner shown, for example, in U.S. Pat. No. 3,163,203, swirls a liquid fuel/air mixture through vane slots, whereas, When the burner is operated on gaseous fuel (natural gas or propane), pressurized air is moved through the vaned slots into a combustion chamber and the gaseous fuel is passed through axially-disposed nozzles where it is then mixed with the swirling air.
Due to the unique problems associated with the production of asphalt, however, these burners and others constructed specifically for the asphalt production operation are unduly complicated in their constructional features and do not perform satisfactorily under all conditions. They also lack other advantages and features such as the ability to provide increased turn down at low fire and extremely stable and intense combustion throughout the burner's firing range in a simple way so as to reduce emissions without, for instance, the need for a compressed air source. At the same time, we have found that the known burners used in the asphalt industry do not satisfactorily enhance and protect the base of a flame recirculation zone or prevent the quenching of the base at that recirculation zone under oil flame.
Furthermore, we have found that the burners currently available do not overcome the foregoing disadvantages while also protecting and shaping the flame as at least the burner. In addition, whereas the prior burners used in asphalt production are known for use with refractory burner block or for use in a refractoryless application, these burners do not provide a satisfactory arrangement for use with and without refractory burning block depending on application temperature and thermal oxidation.
An object of the present invention is, therefore, to provide a new burner and burning method which provides more complete mixing of fuel and air in contrast with the known burners in which only a portion of the air, about one-third of the total volume, has the fuel injected thereinto.
Another object of the present invention is to provide more complete mixing of the fuel and air to obtain more rapid combustion for reducing the overall burner size and lowering CO emissions in a given combustion space before the flame leaves the combustion zone of the dryer. Rapid combustion is used here as combustion intensity defined as the BTU output per hour divided by the combustion space.
Yet a further object of the present invention is to provide a burner which uses swirl to encourage internal recirculation, to promote more rapid and complete combustion and to achieve NOX levels of lowest possible amount with very high combustion intensity and low O.sub.2 levels.
Still a further object of the present invention is to provide a burner which produces a lower noise level and which will run smoother with less resonance in the duct work and drums due to a stable flame and less pulsing.
A further object of the present invention is to provide a burner which requires lower horsepower than previous burners of the same BTU capacity.
A still further object of the present invention is to provide a burner which can be adapted to industrial and high temperature applications where optional refractory burner tile is used for use in refractory lined combustion chambers such as incinerators.
Still another object of the present invention is the provision of a burner having a wider flame than previously obtained which is particularly advantageous for end users in the production of asphalt type large diameter drums.
These objects have been achieved in accordance with the present invention by the provision of a total air burner in which all the air passes through adjustable spin vanes, and the fuel is injected into the entire airstream rather than separating combustion air into two different streams with the fuel injected into only a portion thereof.
Another feature of the present invention is that it produces a wider flame than conventional asphalt burners with the same firing lengths at 50% and 100% firing. This has an advantage over narrower and longer flames of known burners for customers that have large diameter drums.
As a result of the foregoing, a new burner has been produced that is less costly than previously available burners due to its greatly simplified constructional principles while achieving complete combustion and flame stability. Because the burner in accordance with the present invention is inserted only slightly into the drum, it can run with a cooler drier breach plate. Furthermore, the burner in accordance with the present invention uses less horsepower than open fired burners of similar BTU capacity and can be used also in industrial and high temperature applications with refractory burner tile in refractory-lined combustion chambers such as incinerators.